1. The Technical Field
The present invention relates to gas fired burners, of the type which may be used in industrial furnaces and the like, and specifically to burners of the partially premixed type.
2. The Prior Art
Burners which are used in chemical and manufacturing processes often suffer from the problem of matching the heat flux produced by the burner and placed into the space to be heated in the furnace or heat exchanger to the actual load required in order to maximize the amount of heat flux which is being efficiently used, to further maximize the actual rate of production or rate of the process and to reduce problems such as coking, in process heaters for refineries, for example.
Such burners, may also occasionally suffer from operational drawbacks, such as instability of the flame relative to the flame holder, which may be evidenced in terms of lift off of the flame from the burner tile, or flame noise and pulsation. In addition, such burners may often produce undesirable levels of emissions, particularly oxides of nitrogen.
Many conventional gas-fired burners use a diffusion flame combustion process in which combustion occurs over a range of equivalence ratios, including high temperature, lean regions where thermal nitrogen oxides (NO.sub.x) form. One known method for reducing peak flame temperatures is to use a combustion process which creates a fuel-rich primary combustion region and subsequent air staging with corresponding heat loss, resulting in lowering the overall combustion equivalence ratio to achieve complete combustion.
Another known method for reducing peak flame temperatures relates to a combustion process that operates with a fuel-lean primary combustion region and fuel staging in order to raise the equivalence ratio. However, such known methods of stages fuel combustion rely upon a diffusion flame to produce the lean primary stage. External flue gas recirculation has been added to such known methods for further reducing NO.sub.x.
In the combustion of gaseous fuels, NO.sub.x is formed primarily through fixation of molecular nitrogen and oxygen in the combustion air. It is known that thermal NO.sub.x formation depends on the existence of flame regions with relatively high temperatures and excess oxygen. Many conventional combustion methods for reducing NO.sub.x are based upon avoiding such conditions.
It is necessary to consider the prompt NO.sub.x formation process in order to reach very low NO.sub.x levels. Reactions between hydrocarbon fragments and molecular nitrogen can lead to the formation of bound nitrogen species, such as hydrogen cyanide (HCN), which can subsequently be oxidized to nitrogen monoxide. Such processes become significant relative to the thermal mechanism under moderately fuelrich conditions at relatively low temperatures. Avoiding such conditions can reduce prompt NO.sub.x contributions.
Additionally, these prior art burners often employ pilot flames for establishing the primary flame region over the burner in a furnace. The pilot, even though small in heat release may substantially contribute to overall burner emissions, particularly of oxides of nitrogen, under ultra low NO.sub.x operation.
An object of the present invention is to provide a burner which has greatly reduced emissions, particularly of oxides of nitrogen.
Another object of the invention is to provide a burner system which is capable of enabling active management and variation of the heat flux in order to allow for the optimization of the heating process and modify the heat flux of the burner to avoid process shutdowns, while maximizing furnace availability.
A further object of the invention is to provide a pilot for a burner, such as may be used in chemical plant process heaters and the like, which provides the establishment of the primary flame region while contributing less to the heat released by the burner and contributing less to the emissions produced by the burner, particularly oxides of nitrogen.
Still another object of the invention is to provide a gaseous fuel burner system which provides a well organized flame with no significant regions of lean high temperature conditions, which are known to contribute to increased NO.sub.x emissions.
These and other objects of the invention will become apparent in light of the present specification, including claims, and drawings.